Quiz: Electrical Engineering 21 Apr 2021

Quiz: Electrical Engineering
Exam: DSSSB-JE
Topic: Miscellaneous

Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute

Q1. In an ideal transformer the no load primary current I₀ __:
(a) is in phase with V₁
(b) leads V₁ by 90°
(c) out of phase with V1
(d) lags V₁ behind 90°

Q2. A coil of 0.5 H carries a current of 1 A. if the current is reversed in 1 millisecond, emf induced in the coil is:
(a) 100 V
(b) 500 V
(c) 1000 V
(d) 5000 V

Q3. Consider the following statements.
Statement-I: The main function of a starter in a 3-phase induction motor is to provide starting torque.
Statement-II: The 3-phase induction motor is a self-starting motor.
which of the following statement(s) is/are correct?
(a) I only
(b) II only
(c) Both I and II
(d) None of the above

Q4. The mutual flux in a loaded transformer can be varied by varying
(a) Primary current
(b) Secondary current
(c) Load impedance
(d) Reluctance of the magnetic path

Q5. Deep bar rotors are used in induction motors to increase
(a) Full load torque
(b) Full load current
(c) Starting torque
(d) Starting current

Q6. What can be the approximate efficiency of a 3-phase, 50 Hz, 4-pole induction motor, which is running at 1470 rpm?
(a) 92 %
(b) 95 %
(c) 96 %
(d) 98 %

Q7. The armature current in a D.C. shunt generator is given as
(a) (Eg+V)/Ea
(b) Eg/Ra
(c) (Eg-V)/Ea
(d) I_L+I_sh

Q8. The residual magnetism of a D.C. shunt generator can be regained by
(a) Connecting the shunt field to the battery
(b) Running the generator on no load for some time
(c) Grounding the shunt field
(d) Reversing the direction of rotation of the generator

Q9. Which device is used in sub-stations to improve the power factor?
(a) Synchronous reactor
(b) Series inductor
(c) Synchronous condenser
(d) Series capacitor

Q10. Ferranti effect in overhead line is experienced when?
(a) Sending end voltage is more than receiving end voltage
(b) Sending end voltage is less than receiving end voltage
(c) Sending end voltage is equal to receiving end voltage
(d) Voltage at all points in line is equal.

SOLUTIONS

S1. Ans.(d)
Sol. for an ideal transformer, only magnetizing component is taken into amount. Io only inductive nature.

So, Io lags V_1 by 90° for ideal transformer.

S2. Ans.(c)
Sol. e=L di/dt=0.5×(1-(-1))/(1×10^(-3) )=1000 V

S3. Ans.(b)
Sol. A 3-phase induction motor is a self-starting motor. Starter is employed to limit the starting current (not producing starting current).

S4. Ans.(d)
Sol. Mutual flux(𝟇) α 1/reluctance

S5. Ans.(c)
Sol. Deep bar rotors are used in induction motors to increase the starting torque.
Deep bar rotors provide:
More resistance at starting
Low resistance at the running condition

S6. Ans.(d)
Sol. Ns=120f/P=(120×50)/4=1500 rpm
∴efficiency(η)=Nr/Ns =1470/1500×100=98 %

S7. Ans.(d)
Sol.

I_a = I_sn + I_L

S8. Ans.(a)
Sol. To restore residual magnetism, it is necessary to voltage build up, which can be done by connecting shunt field to the battery.

S9. Ans.(c)
Sol. for power factor improvement, synchronous condenser is used.
Synchronous condenser is basically a synchronous motor operating at no-load and leading power factor.

S10. Ans.(b)
Sol. Ferranti effect in overhead transmission lines is experienced at light load condition in this phenomenon, V_R>V_S ie, receiving end voltage is greater than sending end voltage. To overcome this problem, shunt reactors are used at receiving end of lines.

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